The present invention relates to a pressure detecting apparatus for detecting a pressure in a pressure chamber, and more particularly to a tire air pressure detecting apparatus for vehicles.
As one of pressure detecting apparatuses, a tire air pressure detecting apparatus is known as disclosed in JP-A-4-8609. In this conventional apparatus, an axial member is operatively coupled to an elastic member which expands and contracts in response to air pressure in a vehicle tire and is supported by a bearing to be axially movable in correspondence to expansion and contraction of the elastic member. A rotor magnet of which upper and lower halves are respectively magnetized to S-pole and N-pole is rotatably mounted around the outer periphery of the axial member. The axial member, bearing and rotor magnet are housed within a housing, and a first and second magnets are fixed adjacently to each other on the outer periphery of the housing in such a manner that respective magnetic poles thereof facing the magnetic poles of the rotor magnet become S-pole and N-pole.
The rotor magnet is so arranged to be positioned closely to the first magnet when the tire air pressure is kept normal and attraction force F1 between the N-pole of the first magnet and the S-pole of the rotor magnet is so set as to be larger than repulsion force F2 between the S-pole of the second magnet and the S-pole of the rotor magnet. Thus, the rotor magnet is held in a stationary condition.
When the tire air pressure is reduced to the extent that the sum F1+F2 exceeds operation force of the elastic member and the axial member is moved to the position that the rotor magnet is close to the second magnet, the attraction force F1 becomes smaller than the repulsion force F2. This repulsion force F2 causes rotation force to the rotor magnet to rotate the rotor magnet.
It may be detected whether the tire air pressure is normal or abnormal by detecting changes in the position of magnetic poles of the rotor magnet caused by the rotation of the rotor magnet.